Conventionally, in an image forming apparatus employing an electrophotographic method, a corona discharge type charging device has been frequently used for (i) a charging device for uniformly charging a photoconductor or the like; (ii) a transfer device for electrostatically transferring a toner image, formed on a photoconductor or the like, onto recording paper or the like; or (iii) a removing device for removing recording paper or the like from a photoconductor or the like that is electrostatically in contact with the recording paper.
Generally used as such a corona discharge type charging device is either a corotron or a scorotron. The corotron includes (i) a shield having an aperture section facing a charging subject such as a photoconductor or recording paper, and (ii) a discharging electrode provided in the shield and having a line-like shape or a saw-tooth-like shape. The colotron applies a high voltage to the discharging electrode so as to cause corona discharge, with the result that the charging subject is uniformly charged. The scorotron includes a grid electrode provided between a discharging electrode and a charging subject, and applies a desired voltage to the grid electrode, with the result that the charging subject is uniformly charged. See Citation 1.
FIG. 18 is an explanatory diagram schematically illustrating a charging mechanism in such a conventional corona discharge type charging device. As described above, the corona discharge type charging device includes (i) a charging electrode 101 having a line-like shape, a saw-tooth-like shape, or a needle-like shape, and (ii) a counter electrode (discharging target) such as either an image carrier 102 on which a toner image 104 is to be formed or a grid electrode 103. Examples of the image carrier 102 include a photoconductor and an intermediating retransfer member. By applying a high voltage between (i) the discharging electrode 101 having such a small curvature radius and (ii) the counter electrode (discharging target), a non-uniform electric field is formed between the two electrodes. A strong electric field formed in the vicinity of the discharging electrode 101 causes local ionization activity, with the result that electrons are emitted (electric discharge due to electron avalanche). With this, the charging subject such as the photoconductor, the intermediating member, the toner image, or the like can be discharged. Further, the grid electrode 103 is provided so as to control an amount of electrons heading for the charging subject such as the image carrier 102. Therefore, the grid electrode 103 is also subjected to the discharging of the electrons.
Further, for example, each of Citations 2 and 3 discloses that such a corona discharge type charging device as described above is used for a pretransfer charging device for charging a toner image that is to be transferred to a transfer medium such as an intermediating retransfer member or recording paper. With the technique each described in Citations 2 and 3, a charge amount of the toner image formed on the image carrier can be uniformized before the transfer, even when there is fluctuation in the charge amount of the toner image. This makes it possible to restrain decrease of a degree of margin in transferring the toner image, so that the toner image is stably transferred onto the transfer medium.
However, each of the aforementioned conventional charging devices suffers from a plurality of problems.
A first problem is as follows. That is, the conventional corona discharge type charging device generates a large quantity of discharge products such as ozone (O3) and nitroxide (NOx). Specifically, a nitrogen molecule (N2) is separated into nitrogen atoms (N), and each of the nitrogen atoms is combined with an oxygen molecule (O2), with the result that nitroxide (nitrogen dioxide: NO2) is generated. Likewise, an oxygen molecule (O2) is separated into oxygen atoms (O), and each of the oxygen atoms is combined with an oxygen molecule (O2), with the result that a large quantity of ozone (O3) is generated.
The large quantity of ozone thus generated causes problems such as (i) generation of ozone odor, (ii) adverse effect on human body, and (iii) deterioration of parts due to strong oxidation. Further, the nitroxide thus generated is adhered to the photoconductor in the form of ammonium salt (ammonium nitrate). This will be a cause of formation of an unnatural image. Especially, such ozone and NOx easily causes a defective image in an organic photoconductor (OPC) normally used in such a charging device. Examples of the defective image include: formation of a white spot in an image and image deletion.
Further, the nitroxide is adhered to the grid electrode of the corona discharge type charging device, with the result that the surface of the grid electrode is oxidized and is corroded. This causes secondary generation of insulative metal oxide on the grid electrode, with the result that the uniformity in the charging is spoiled. Such non-uniformity in the discharging causes image deterioration. This is problematic.
Thus, in an intermediating transfer type color image forming apparatus having a plurality of transfer portions, such a problem in the respective quantities of the produced ozone and NOx makes it difficult to provide the pretransfer charging device in the upstream with respect to all the transfer portions (primary and secondary transfer portions), even though it is preferable to provide the pretransfer charging device therein.
Further, the photoconductor requires a charging device for use in latent image formation. Therefore, in consideration of an adverse effect on the photoconductor, it is difficult to provide the pretransfer charging device in addition to the charging device so as to carry out pretransfer charging with respect to a toner image formed on the photoconductor. An only way to avoid this problem is to use an amorphous silicon photoconductor, which can be charged positively and therefore allows generation of a relatively small quantity of ozone, which is excellent in its plate life, and from which a discharge product is forcefully removable.
Meanwhile, a contact charging method using a conductive roller or a conductive brush as a charging device for directly charging a photoconductor has been adopted in recent years for the purpose of reducing generation of ozone. However, in the contact charging method, it is difficult to charge the photoconductor without distorting the toner image. Accordingly, the conventional non-contact corona discharge type charging device is used for the pretransfer charging device. However, by providing the conventional corona charging type pretransfer charging device in the image forming apparatus including the charging device adopting the contact charging method, such a feature of the contact charging method that allows for reduction of ozone is eliminated.
A technique for reducing a quantity of produced ozone is disclosed in, e.g., Citation 4. Specifically, Citation 4 discloses a charging device, including (i) a large number of discharging electrodes arranged with a substantially constant pitch therebetween in a predetermined axis direction; (ii) a high-voltage power supply for applying a voltage, to each of the discharging electrodes, a voltage equal to or higher than a discharge threshold voltage; (iii) a resist member provided between an output electrode of the high-voltage power supply and the discharging electrode; (iv) a grid electrode provided between the discharging electrode and a charging subject so as to be adjacent to the discharging electrode; and (v) a grid power source for applying a grid voltage to the grid electrode. A gap between the discharging electrode and the grid electrode is set at 4 mm or less such that a discharging current is reduced and the quantity of produced ozone is accordingly reduced.
The technique of Citation 4 allows reduction of the quantity of produced ozone, by reducing the discharging current. However, the quantity of the reduction of ozone is still insufficient, i.e., ozone is generated at approximately 1.0 ppm. Further, Citation 4 suffers from another problem: discharging is unstable due to (i) adhesion of a discharge product, toner, bits of paper, and the like to the discharging electrode, and/or (ii) abrasion and deterioration of the tip of the discharging electrode. The abrasion and the deterioration are caused due to discharging energy.
Further, the gap is narrow between the discharging electrode and the charging subject, so that charge non-uniformity is caused with ease in the longitudinal direction (pitch direction of the plurality of discharging electrodes) due to the pitch between the discharging electrodes. A conceivable way to eliminate the charge non-uniformity is to reduce the pitch between the discharging electrodes. However, this causes increase of the number of discharging electrodes, thereby increasing manufacturing cost.
A second problem of the conventional charging device is corona wind (also referred to as “ozone wind”). As indicated by an arrow 105 in FIG. 18, the corona wind blows from each of the discharging electrodes to the charging subject due to the flow of the electrons, which flow is caused by the corona discharge. Accordingly, the corona wind distorts the toner image 104 formed on the image carrier 102, in cases where the conventional corona discharge type charging device is used for the pretransfer charging device.
Citation 1: Japanese Unexamined Patent Publication Tokukaihei 06-11946/1994 (published on Jan. 21, 1994)
Citation 2: Japanese Unexamined Patent Publication Tokukaihei 10-274892/1998 (published on Oct. 13, 1998)
Citation 3: Japanese Unexamined Patent Publication Tokukai 2004-69860 (published on Mar. 4, 2004)
Citation 4: Japanese Unexamined Patent Publication Tokukaihei 08-160711/1996 (published on Jun. 21, 1996)
Citation 5: Japanese Unexamined Patent Publication Tokukai 2005-316395 (published on Nov. 10, 2005)